Rf and if digitization in Radio Receivers: Theory, Concepts, and Examples

The key factors in radio receivers where digitization occurs at the IF or RF are analog-to-digital 

conversion and digital signal processing. Although not discussed in this paper, digital-to-analog 

conversion is also a factor for applications requiring analog output (such as voice). Because of 

the rapid advances in hardware development of ADCs, DACs, digital signal processors, and 

specialized ICs, development of radio receivers using digitization at the IF (and in some cases at 

the RF) is becoming increasingly popular.  

Hardware limitations of ADCs, digital signal processors, and DACs place constraints on 

digitization at the IF and RF in radio receivers. It was shown that digitization at the RF, in 

general, requires some type of bandlimiting (filtering) and amplification before the actual 

digitization takes place. The required amount of filtering and amplification is application-

specific. ADC performance is improving rapidly. However, there is a tradeoff; one can get either 

high sampling rates or high resolution, but not both simultaneously. Therefore, the high sampling 

rate ADCs required for wide bandwidth applications may not have sufficient SFDR. Digitization 

at the RF is now being considered for satellite receivers since a large SFDR is not a necessity, 

very high sampling rate ADCs already exist, and even faster ADCs are being developed. For 

receiver applications requiring a large SFDR, such as HF communications, digitization at the IF 

is currently a more practical option.  

Digital signal processors may present an even greater limitation than ADCs in radio receivers 

using digitization at the RF or IF. The speed, size, and cost of these processors are important for 

a particular radio receiver application. The requirement for real-time operation for many radio 

receivers places a heavy burden on these processors. It is difficult to discuss limitations of digital 

signal processing in general terms since many algorithms can be implemented in radio receivers 

depending on the specific application. The amount of time that signal processing requires is a 

function of the bandwidth of the signal, the speed of the processor, and the number and 

complexity of the algorithms required to perform the needed radio receiver functions.  

Several potential methods and devices that are expected to be useful in radio receivers employing 

digitization of the RF or IF are discussed in this report. Various types of quantization techniques 

include uniform, μ-law, adaptive, and differential quantization. These techniques can be used to 

improve the dynamic range of ADCs as well as make the SNR insensitive to input signal 

amplitude. Nonlinear compression devices such as log amplifiers and automatic gain control 

amplifiers also can be used to improve the dynamic range of ADCs. Postdigitization algorithms 

for improving the SFDR provide extended dynamic range for presently available ADCs. 

Sampling downconverters, based on the theory of bandpass sampling, may be coupled with ADC 

technology to provide improved ADC performance for bandpass sampling applications. 

Specialized ICs for digital signal-processing tasks required in radio receiver applications show 

great potential in increasing processing speed; this allows more receiver functions to be executed 

in real time. Bandpass sampling is expected to be important for radio receiver applications since 

it allows the use of more readily available ADCs with lower sampling rates and higher SFDR.  

This report provides information on receivers implementing digitization at the RF or IF. The 

topic of EMC analysis for these radio receivers needs to be investigated further. Due to the 

tremendous variety of possible receiver implementations employing digitization at RF or IF, 

 

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